Photoconductive elements, also called photoreceptors, are composed of a conducting support and at least one photoconductive layer which is insulating in the dark but which becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge in such areas. The resulting charge pattern on the surface is referred to as an electrostatic latent image. The latent image can be made visible by application of a liquid or dry developer containing finely divided charged toner particles which, if desired, can be transferred and fixed to another surface such as a sheet of paper.
Numerous photoconductive materials have been described as being useful in electrophotography. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines and the like. Especially useful are aggregate photoconductive compositions that have a continuous electrically insulating polymer phase containing a finely divided, particulate co-crystalline complex of at least one pyrylium-type dye salt and at least one polymer having an alkylidenediarylene group in a recurring unit.
Aggregate compositions used in photoreceptors can be prepared by several techniques, such as, for example, the "dye first" technique described in Gramza et al., U.S. Pat. No. 3,615,396, incorporated herein by reference. Alternatively, they can be prepared by the "shearing" method described in Gramza, U.S. Pat. No. 3,615,415, incorporated herein by reference. This latter method involves the high speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment, as disclosed in Light, U.S. Pat. No. 3,615,414, referred to hereinafter. By whatever method prepared, the aggregate composition is applied with a suitable liquid coating vehicle onto a support or underlying layer to form a separately identifiable multiphase aggregate composition, the heterogeneous nature of which is generally apparent when viewed under magnification, although such compositions may appear to be uniform to the naked eye in the absence of magnification. There can, of course, be macroscopic heterogeneity. Suitably, the pyrylium type dye-salt-containing aggregate in the discontinuous phase is finely-divided, i.e., typically predominantly in the size range of from about 0.01 to about 25 .mu.m.
Photoconductive elements can comprise single or multiple active layers. In a single layer photoconductive element charge generation and charge transport take place within the same layer. Single active layer aggregate photoconductive elements are described in Light, U.S. Pat. No. 3,615,414 and in Gramza et al., U.S. Pat. Nos. 3,732,180 and 3,615,415. Contois and Rossi, U.S. Pat. Nos. 3,873,312 and 3,873,311 describe the use of aggregate photoconductive compositions and elements containing organic photoconductors with a styrylamino structure. Berwick et al., U.S. Pat. No. 4,175,960, describes a multi-active photoconductive element having an aggregate charge generation layer.
Single active layer aggregate photoconductive compositions have found many commercial applications. They are easily and inexpensively manufactured and have the additional advantage of being able to photoconduct to either a negatively or positively charged surface.
A single active layer aggregate Photoconductive composition intended for coating in a drum format differs in certain respects from one intended for web coating. In particular, drum coating requires the use of higher boiling solvents in the formulation to promote good coating uniformity. However, because the coating substrate is mounted in a vertical position during coating, drying conditions must be carefully controlled to maintain end to end thickness uniformity. Formulations for drum coatings typically contain 1,1,2-trichloroethane as a solvent, and coated drums are generally dried in a convection oven set at a temperature of about 90.degree. C. to volatilize this solvent. It would be desirable to employ higher temperatures to remove the solvent completely, but it has been found that drying temperatures above 100.degree. C. have a detrimental effect on the electrophotographic speed of a single active layer aggregate photoconductor.
Essentially solvent-free photoconductive drum coatings are desirable for several reasons. First, residual solvent is capable of plasticizing a coating, adversely affecting its physical properties and diminishing its durability. In addition, the presence of a small amount of residual solvent that is slowly volatilized with continuing use of the coating can lead to unstable electrophotographic performance.
In commercial electrophotographic copying machines, especially those of compact design where drum photoconductors are typically employed, internal temperatures increase greatly, as much as 70.degree. C., during a long run job. Because of the heat capacity and thermal conductivity of aluminum, the metal most commonly used for drum substrates, the photoconductive layer quickly equilibrates with the hot environment. Continued drying, with consequent change in the electrophotographic performance of the single active layer aggregate photoconductor, is the result.
Thus, a need exists for single active layer aggregate photoconductors, particularly in a drum format, which can be dried at a high enough temperature, at least about 100.degree. C., and preferably from about 110.degree. C. to 145.degree. C., to ensure complete removal of coating solvent and form a photoconductive layer with good physical properties and excellent, stable electrophotographic performance.